1. Field of the Invention
This invention relates generally to fixed cutter rotary drag bits for earth boring, and more particularly to improvements in bit design. Specifically, this invention relates to the design of stud type carrier elements inserted into the body of a drill bit to support cutting elements mounted on the carrier elements.
2. State of the Art
Fixed cutter rotary drag bits for subterranean earth boring have been employed for decades. Fastened to the bottom of a rotating drill string, a drag bit chips, shears, or plows the earth formation ahead of it, the formation debris or cuttings flowing upward in an annular column of drilling fluid or "mud," surrounding the drill string. Mud is typically injected through nozzles in the bit face to cool and clean cutting surfaces of cutting elements on the bit face and to carry away the cuttings up the well bore annulus.
The bit body is typically of steel or of a matrix of tungsten carbide, the former type being usually forged or cast, while liquid infiltration powdered metal matrix metallurgy is generally employed in the latter. Finish machining of either type bit body may be performed by various methods known in the art, as may hardsurfacing of the bit face, depending on material properties of the body.
Inserts called studs are fixed to the bit body. The studs comprise a carrier element and a cutting element. The carrier element's function is structural and the cutting element's function is to chip, shear, or plow material from the earth formation being drilled by the bit. The carrier elements are secured by interference fit, threads, welding, brazing or other means in openings provided for them in the face of the bit body. Buttresses on the bit body often back up the carrier elements to add support. The studs thus protrude in rows or arcuate arrays extending from near the center radially across the face of the bit body to the gage and usually for some axial distance, many bits having conical or parabolic profiles. The cutting elements, usually brazed to the carrier elements, typically are polycrystalline diamond compacts ("PDCs") (sometimes called preforms) comprised of a cutting face of diamond bonded during manufacture to a layer of tungsten carbide.
Prior Art:
U.S. Pat. Nos. 4,199,035; 4,200,159; 4,350,215; 4,351,401; 4,382,477; 4,398,952; 4,484,644; 4,498,549; 4,505,342; 4,593,777; 4,705,122; 4,714,120; 4,718,505; 4,749,052; 4,877,096 and 4,884,477 address the configurations, manufacture, utility, and governing considerations of matrix bits. The foregoing patents are incorporated by reference here for their teachings of cutting elements, carrier elements and matrix bits using them.
U.S. Pat. No. 4,199,035 (Thompson, 1980) discusses a method of threadedly attaching a stud in a bit body. The patent discusses the construction of a compact, a cluster of abrasive particles or crystals bonded together either by self bonding or bonding by means of a medium disposed between the crystals or some combination of both methods. Noting the large variety of dynamic loads to which cutting elements are exposed during drilling, the patent identifies the importance of repair of individual cutters in a bit. The patent points out the impracticality of repairing permanently mounted cutters.
U.S. Pat. No. 4,200,159 (Peschel et al., 1980) discusses the technique of making carrier elements having cutting elements mounted on them separately from the bit body. The patent also discusses the difficulty of forming the diamond materials in situ together with the bit body due to thermally-induced diamond degradation and lack of replaceability of broken cutting elements, giving rise to the need for a stud-type bit.
U.S. Pat. No. 4,350,215 (Radtke, 1982) discusses the manufacture of drill bits, including the formation of a bit body with pockets into which the cutting elements are brazed.
U.S. Pat. No. 4,351,401 (Fielder, 1982) discusses a matrix drag bit using diamond preform cutters mounted on studs positioned in sockets in the face of the bit. The patent discusses the advantage of cutters arranged on studs in the face of the bit for maintaining compression on the cutters rather than tension due to bending forces. This highlights the importance of avoiding bending since materials with low toughness may fail precipitously in tension. Also, the patent discusses the value of being able to replace a single preform which has been damaged rather than having to salvage the entire bit. That is, it is much more economical to salvage a bit by repairing a damaged preform, stud, etc. rather than having to destroy the bit to recover all of the preforms having useful life remaining.
U.S. Pat. No. 4,382,477 (Barr, 1983) discusses the use of "preform" cutting elements made with diamond facing on a backing layer of tungsten carbide which is mounted on a support member mounted on a drill bit. The patent discusses at length the variety of stresses experienced by the preform and the importance of believing the various stresses. Among the difficulties are the increased friction on the formation due to having a hardened underlying supporting material behind the preform. Likewise, the resulting heat weakens braze. Perhaps most importantly here, the '477 patent discusses the deformation which the preform must undergo due to deformation of the underlying support member and underscores the need for resilience of cutters.
U.S. Pat. No. 4,398,952 (Drake, 1983) discusses a method for forming rolling cutter bits. The method involves providing a first powder mixture comprising mainly a refractory with a minor proportion of binder metal. A second powder comprises a powder binder metal with the powder refractory material in a lesser proportion than the first powder. The method involves mixing the powders in differing proportions starting with a majority of the first powder (giving rise to harder material) and eventually at the inner most region of a mold having a nearly 100% composition of the second powder. The result is a gradient in the roller cutter composition once the mold filled with the powdered mixture is sintered.
U.S. Pat. No. 4,484,644 (Cook et al., 1984) discusses a powder metallurgy technique of making steel and tungsten carbide forgings with a 100% density and having a hardness gradient along the length of the foregoing. The articles so formed can serve as the inserts or studs in rock cutting bits.
U.S. Pat. No. 4,498,549 (Jurgens, 1985) discusses drill bit cutting structures comprising segments of PDCs bonded with adjacent blanks to carrier elements.
U.S. Pat. No. 4,505,342 (Barr et al., 1985) discusses drag-type well drilling bits. The patent discusses the use of PDCs mounted on studs inserted into a bit body to form a bit. The patent also discusses the difficulties of cooling, integrity, and the cracking and shearing of the studs as well as the need for resilience in the bit body.
U.S. Pat. No. 4,593,777 (Barr, 1986) discusses at length the importance of the orientation of the cutting face of a drill bit compared to the formation which is being drilled. The patent discusses at length the importance of rake angle, the angle formed by the cutting edge and the formation, in achieving rate of penetration (ROP) in various types of formations. The patent also discusses some of the trade-offs between maximum ROP in soft formations and maximum wear in hard formations without having to extract the drill string from the hole in order to change drill bits. The patent also discusses the tradeoff of material properties between the various components of a drill bit using stud-type cutting elements.
U.S. Pat. No. 4,705,122 (Wardley et al., 1987) discusses a preform cutting element comprising a circular tablet having a polycrystalline diamond face bonded to a backing layer of tungsten carbide mounted on a stud inserted in a bit body. The stud is basically cylindrical. This classic geometry is common to the industry. However, the patent does highlight the need for proper orientation of the cutting face of the cutting element and the need for an open area in front of the cutting face for carrying away debris. In addition, it discloses the need for support in the stud for the dynamic loads applied to the cutting element and the surface of the stud.
U.S. Pat. No. 4,714,120 (King, 1987) discusses a scheme to make cutters in pairs along the crown of a matrix-type bit body to make the cutting elements less susceptible to gross failure by shearing.
U.S. Pat. No. 4,718,505 (Fuller, 1988) discloses an abrasive element which follows a cutting element in a matrix bit using studs, in the event of the failure of a stud. The patent identifies the need to maintain some ability to cut in the event of failure or excessive wear of the principal cutting edge of a cutting element mounted on a carrier element (stud).
U.S. Pat. No. 4,749,052 (Dennis, 1988) discusses the placement of round cross-sectional studs into recesses in the face of a drill bit for attachment by press-fit or brazing.
U.S. Pat. No. 4,877,096 (Tibbitts, 1989) discusses a replaceable stud cutter for use in matrix drag bits. The patent discusses the prior art practice of destroying an entire bit body when cutters are worn in order to recover or salvage diamond cutters for future use on other bits. Likewise, since some cutters on a bit may be damaged while others are in useful condition, the -096 patent addresses the issue of cutter replacement to extend the life of a bit.
U.S. Pat. No. 4,884,477 (Smith et al., 1989) discusses the construction of a rotary drill bit of the metal matrix type having cutting elements mounted on its exterior. The patent discusses providing a rotary drill bit which has at least some portion of its construction of the metal matrix made of tungsten carbide. Provision of a substitute filler material mixed with the tungsten carbide improves the toughness of the bit. A technique of hardfacing such tougher bits for enhanced abrasion and erosion resistance is also disclosed.
Stud-type carrier elements are generally of harder and stronger materials than the bit body and can resist abrasion from the formation and its resulting debris and erosion from solids-laden drilling mud. Harder materials often have low toughness but high strength, thus supporting high stresses, so long as their surface integrity remains. That is, even for strong materials, low toughness may cause fractures to progress through a member rapidly once outermost surfaces are compromised by minute cracks.
However, the ultimate strength of a high toughness material is typically reached after absorption of substantial energy through plastic strain. Material of low toughness, on the other hand, typically reaches ultimate strength after only slight energy absorption through plastic deformation. The result is that a low toughness material may be very strong and functional while it lasts, but unforgiving of flaws.
Another key factor in the use of hard material of low toughness is the presence of surface defects which cause stress concentrations. Glass demonstrates this phenomenon. Glass free of inclusions and surface defects is strong, supporting substantial loads even in bending. However, when glass is exposed to the atmosphere, airborne impurities etch the glass causing microscopic imperfections or cracks in the surface. Since the glass is so unyielding, stresses resulting in the surface of the glass tend to concentrate in the tiny region at the leading edge of the cracks. Such stress, if not reduced over a broader area through local yielding of the material, maintains the stress concentrations at the leading edge of each of the surface imperfections even as each crack advances in response. The region around the tip of the crack fractures, rather than elongating, applying the stress concentration at the new location of the tip. With the application of additional stress or repeated stress the imperfection advances completely through the material, sometimes very rapidly, eventually fracturing (rupturing) the entire cross section of the material.
Other materials of low toughness behave similarly. Without some ability to permit yielding locally around cracks, total rupture of a section of material can occur rapidly. Given the grinding, chipping, abrading and eroding nature of the drilling environment, surface defects in materials of low toughness can create stress concentrations in studs formed of such materials, which stress concentrations eventually fracture the studs. Thus, unless possessed of high toughness, a hard stud which reduces the effects of abrasion will be more subject to fracture. A tougher material less subject to catastrophic fracture will be more subject to abrasion and erosion. Whether a stud is abraded or eroded, broken away from its brazed position in the bit body or fractured, it is rendered equally useless.
The cutting of an earth formation by a drill bit is actually accomplished by the action of the cutting elements which are attached to faces of the free ends of the carrier elements secured in the bit body. The cutting elements are generally of superhard material such as synthetic diamond, previously referred to herein as polycrystalline diamond compacts or "PDCs," although other materials such as cubic boron nitride have been employed. Polycrystalline diamond compacts (PDCs) are cutting elements having a tungsten carbide substrate on which a diamond face is formed with a catalyzing metal by application of extreme heat and pressure.
Stresses resulting in a stud during operation of a drill bit may include, individually or in combination, bending, shear, tension and compression caused by the earth formation resisting the stud's motion on its cutting (free) end while the bit body drives forward the other (secured) end axially and tangentially with respect to the direction of advance of the drill bit. The stresses occur in different locations and to differing degrees. Also, the extent of a stress varies depending on its type and location.
On the other hand, tensile stress due to bending of an axially inserted, cylindrical carrier element as it supports the cutting element transversely can be very large. That force can also be exacerbated by the stress concentration at the locus of contact between the carrier element and the bit body.
Moreover, as explained above, any material of comparatively low toughness, including some tungsten carbides (WC) will be comparatively unyielding in tension. This characteristic results in a component of low toughness which breaks upon reaching its ultimate stress. However that stress level is more easily reached in the presence of stress concentrations from a change in material cross section at the point of penetration into the bit body, at any stress discontinuity or at a material flaw such as a small crack or notch. As explained above, such stress concentrations enhance propagation of cracks.
By contrast, materials with relatively high fracture toughness such as some steels, high-cobalt-content tungsten carbides, or large-grain-size tungsten carbides, will yield locally under sufficient stress, relieving the stress over a region and thus stopping the propagation of a crack. The high inertia and energy input of a drill string can result in very high dynamic loads. A very high dynamic load of very short duration may cause a fracture. Thus, a surface flaw need not be substantial, or exist for a long time to propagate. Although cracks can propagate slowly across a section over time, they can also propagate instantly. Lower toughness materials tend to fail with more rapid propagation of cracks. In such material, the crack may propagate quickly to catastrophic failure under high stress, such as dynamic loading often imposes.
In bending, the maximum stress in a section symmetric about its neutral axis (typically the centerplane perpendicular to the applied force) is on the outermost fiber. The outermost fiber exists at the outer surface at a maximum distance from the neutral axis. In a cylindrically shaped stud cantilevered from a close fitting penetration in a bit body, for example, bending forces imposed by the cutting face at the free end apply maximum tension at the surface of the stud on the side on which the force is applied. Maximum compression occurs on the diametrically opposite surface at the position where the stud enters the bit body.
A commonly employed stud is a cylindrical rod, for ease of manufacture and to fit in maximum numbers over the surface of a small bit body. The strongest stud materials of maximum toughness (consistent with cost) are desirable. However, materials with relatively high erosion and wear resistance but low toughness are typically used. The stud should extend the maximum distance possible from the surface of the bit body to allow space for chips of debris to pass to prevent clogging or "bailing" of the bit. This configuration, however, creates the highest bending stress. Of course, the cutting edge must be at the furthest extremity of the stud to contact the formation. Preferred sizes and spacing of cutters must actually be balanced against the properties of available materials. Thus, in reality, various shapes and configurations will result as each limiting factor is incorporated in a design. However, the tradeoffs to be made are not always apparent, even with idealized parameters.
A material which minimizes abrasion may have low toughness and thus be susceptible to stress concentrations, stress corrosion cracking, and rapid crack propagation, which undermine its structural integrity. A material which can resist such fracture by its toughness may be easily abraded.
Sources of reduced working stress include the interference fit of a stud into an opening in a drill bit body. Even without press-fitting, for example, if the studs are brazed in holes in the bit face, the difference in the coefficients of thermal expansion of dissimilar metals (stud and bit body) introduces residual stresses after the brazing process as the drill bit cools down.
At the point of stud penetration into the bit body, a change in effective cross section occurs over which stress is spread. This change in cross section causes a stress concentration effect. Both effects can reduce the maximum working load permissible. Residual stress of mounting and the restraint imposed by the bit body may also increase plane stress locally in the stud.
The compressive stresses in the stud will also tend to reduce the maximum tensile stress which the stud can support normal thereto. Thus, the tolerable bending load of a cantilevered stud is reduced when compressive stress is applied, such as by an interference fit.
cutter wear characteristics can, and often do, dictate the useful life of a drill bit. Tremendous costs result if cutters wear out prematurely at the bottom of a drill hole several thousand feet deep, the bit cost itself being a small portion of the total rig time and personnel cost involved in retrieving and replacing the bit in such a circumstance.
The mechanical fracture of even one stud may be even more catastrophic, as such an occurrence can stop a drill bit's progress by failing to cut its share of the formation. Bit replacement is necessary when a missing cutting element leaves an uncut cylinder or annular collar remaining on the formation for the bit to ride upon. Thus, if a stud breaks down for any reason, the bit may eventually stop cutting and merely ride on the uncut formation even if all of the other cutters remain intact and fully functional. Such a failure results in a bit replacement requiring tripping in and out of the hole.
One solution to the problem, to date unaddressed by the prior art, is to manufacture tough studs having a hard surface. In order to create such a stud having maximum fracture toughness with maximum surface hardness, a composite structure having different characteristics across its cross section is desirable. Also, means to reduce stress concentrations due to loading or material flaws is needed.